Abstract

Marine invertebrates with skeletons made of high-magnesium calcite may be especially susceptible to ocean acidification (OA) due to the elevated solubility of this form of calcium carbonate. However, skeletal composition can vary plastically within some species, and it is largely unknown how concurrent changes in multiple oceanographic parameters will interact to affect skeletal mineralogy, growth and vulnerability to future OA. We explored these interactive effects by culturing genetic clones of the bryozoan Jellyella tuberculata (formerly Membranipora tuberculata) under factorial combinations of dissolved carbon dioxide (CO2), temperature and food concentrations. High CO2 and cold temperature induced degeneration of zooids in colonies. However, colonies still maintained high growth efficiencies under these adverse conditions, indicating a compensatory trade-off whereby colonies degenerate more zooids under stress, redirecting energy to the growth and maintenance of new zooids. Low-food concentration and elevated temperatures also had interactive effects on skeletal mineralogy, resulting in skeletal calcite with higher concentrations of magnesium, which readily dissolved under high CO2. For taxa that weakly regulate skeletal magnesium concentration, skeletal dissolution may be a more widespread phenomenon than is currently documented and is a growing concern as oceans continue to warm and acidify.

1. Introduction

Calcifying marine species use a variety of mineralogical forms or ‘polymorphs’ of CaCO3 in skeletal material. These polymorphs include aragonite, calcite as well as calcite minerals containing high concentrations of magnesium (Mg). Many species incorporate more than one form of calcite into skeletal material [1–3]. The compositions of such biogenic calcites vary by taxonomic group and typical levels of Mg substitution into the crystalline lattice range from 0 to 20 mol% MgCO3 [1]. In some species, the Mg content of skeletal calcite can be influenced by abiotic factors, including temperature, light and seawater carbonate saturation state among others [1,4,5]. As the Mg content of calcite increases, these materials become increasingly soluble and therefore more vulnerable to dissolution as seawater pH decreases [1–3].

Ocean absorption of global CO2 emissions has reduced average surface water pH by at least 0.1 pH units since the beginning of the Industrial Revolution [6]. This process, termed ocean acidification (hereafter OA), is projected to accelerate in coming years, with a reduction of 0.3–0.5 pH units expected by 2100 [7]. Organisms secreting skeletons containing high-Mg calcite (i.e. exhibiting more than 12 mol% MgCO3) may be particularly vulnerable to increasing OA because of the kinetic sensitivity of this material to decreasing CaCO3 saturation states in the global ocean [1–3]. Additionally, variation in the chemical and physical properties of biogenic calcites make Mg-dissolution relationships tenuous for some taxa, and chemical stability may not always be a simple function of Mg composition across all groups [1]. Taxa that secrete variable-Mg calcite skeletons in modern seas include for example, coralline algae, serpulid worms, foraminifera, echinoderms and a subset of bryozoan species among others [4].

Studies of the biological effects of OA suggest that environmental factors such as food availability [8,9] and warming [10,11] play critical roles in mediating the susceptibility of marine taxa to acidification. For example, plentiful food can offset the effects of OA [9,12], whereas increasing temperatures can either magnify [13] or ameliorate [14] its effects. There are many physiological mechanisms by which food and temperature might influence the susceptibility of marine organisms to acidification. Skeletal mineralogy is a plastic trait in some marine calcifiers [3], and mineralogy is known to vary with temperature [4,15], while overall calcification rates can be influenced by an animal's energetic state [16]. In this study, we investigate how mineralogical changes driven by both temperature and food availability interact to affect organismal vulnerability to OA.

(a) Ocean acidification in the California Current Large Marine Ecosystem

Along portions of the west coast of North America, wind stress patterns and near shore ocean circulation lead to the upwelling of deeper waters during spring and summer months that are naturally high in CO2 and low in pH [17]. Intertidal and subtidal habitats exposed to these upwelled waters experience low saturation states with respect to aragonite and high-Mg calcite [17–19], and also experience significant spatial and temporal variation in temperature, plankton and nutrient concentrations [20]. This variation creates an alongshore mosaic of dynamic environmental conditions along large stretches of coastline. Furthermore, additional reductions in pH projected for surface waters in the California Current Large Marine Ecosystem (CCLME) due to increasing anthropogenic CO2 emissions are among the most extreme expected for the World's oceans [21]. Models indicate that within as few as 30 years, atmospheric CO2 inputs and altered oceanographic circulation could result in summer-long under-saturation with respect to aragonite and high-Mg calcite in the upper 60 m of the water column [21]. These changes have the potential to impact marine organisms that cannot tolerate extended exposure to low saturation states, with important implications for the ecology of the systems they inhabit [22].

(b) Study species

Bryozoans are a phylum of suspension-feeding colonial marine invertebrates, most of which calcify, with nearly 6000 described species found in benthic marine ecosystems throughout the world [23]. Bryozoans form colonies of genetically identical units termed zooids via asexual budding, and can be experimentally subdivided into clonal replicates and assigned to different treatments [24,25], allowing for experimental designs where variance components can be partitioned among genotypes. Skeletal mineralogy is a highly plastic trait in some bryozoan families, while in others, it appears more constrained [26–28]. Identifying environmental factors that influence skeletal Mg in a given clade can help determine which factors will exacerbate versus ameliorate the negative effects of OA.

Jellyella tuberculata (formerly Membranipora tuberculata [29]) is an encrusting cheilostome bryozoan typically found growing as an epibiont on macroalgae in low-intertidal and subtidal habitats in most of the World's temperate and tropical seas [29,30]. Along the west coast of Central and North America, this species ranges from Peru to northern California, USA. We used a six-month laboratory experiment to test how skeletal mineralogy and colony growth varies in J. tuberculata clones when exposed to projected changes in future ocean temperatures and carbonate chemistry, while simultaneously manipulating food availability. We hypothesized that growth and Mg content in lineages of bryozoan clones would decrease under elevated CO2 and low CaCO3 saturation states, but increase under warmer temperatures. We also expected increased food to buffer growth against the negative effects of high CO2. Finally, we hypothesized that the interactive effects of warming temperature and high CO2 would increase skeletal dissolution, whereas high food availability would reduce dissolution.

2. Material and methods

(a) Collections, cloning and culture protocol

Colonies of J. tuberculata were collected on intact blades of red algae from the low-intertidal zone at Dillon Beach, CA, USA (38°15′20.72 N, 122°58′15.63 W) during November 2013. Colonies were grown off of algal substrata onto acetate sheeting to provide a standardized substrate across all replicates, and were held for four months under pre-treatment conditions at Bodega Marine Laboratory (see the electronic supplementary material for further culturing methodology). While interactions between epiphytic bryozoan colonies and algal substrata are possible [31], examining such interactions was beyond the scope of this study. Colonies were propagated to produce eight replicate clones of each of eight genotypes, resulting in a total of 64 experimental colonies. One clone from each genetic replicate was then randomly assigned to each of the eight treatment combinations (see Experimental design). These colonies were cultured under approximately 350 µatm CO2 at 16°C at high food concentrations and then photographed under a stereo microscope (Leica M125 with a Leica DC290 camera; Leica Microsystems, Wetzlar, Germany) prior to exposure to the experimental treatments (hereafter referred to as the treatment phase). Colonies were then photographed again at the completion of the eight-week treatment phase.

Temperatures and CO2 values were chosen to represent typical high and low values encountered by J. tuberculata along the California coast, ranging from non-upwelling summer conditions in southern California to upwelling conditions along the northern California coast [18,19,33]. We also report aragonite saturation state values of treatment seawater (Ωaragonite), a measure of the thermodynamic potential for this form of CaCO3 to precipitate or dissolve in solution, which is dependent on both the temperature of seawater and dissolved Ca2+ and CO32− concentrations. Because temperature-dependent solubility relationships for biogenic Mg-calcites are poorly resolved in most taxa due to carbonate and cation disordering [1], we use the established temperature-dependent solubility relationship of aragonite as a proxy for the solubility of J. tuberculata calcite. Recent syntheses have mapped the solubility of aragonite to the solubility of some biogenic high-Mg calcites with molar Mg to Ca ratios (Mg/CaC) of approximately 0.12–0.14 [2,3,34]. In this study, calcification with observed Mg/CaC at or exceeding 0.12 was therefore assumed to meet or exceed the solubility of aragonite, and hence be potentially susceptible to dissolution at Ωaragonite ≤ 1.

(c) Skeletal magnesium analyses

We analysed the Mg content in skeletons of autozooids (feeding zooids) generated during the treatment phase (hereafter referred to as treatment phase zooids) in three to four colonies from each of our eight treatment groups. We also analysed the Mg content of zooids grown at 16°C during the pre-treatment phase (hereafter referred to as pre-treatment zooids) in one of these same colonies. The latter analysis was performed to compare the effects of treatments on the Mg content of calcification that was initially accreted during benign common garden conditions. This analysis was intended to provide information on how the Mg/CaC of calcification generated under one set of conditions may be affected by subsequent changes in conditions experienced by colonies.

Elemental analyses were carried out using an electron microprobe (Cameca SX-100, Gennevilliers, France). Bryozoan colonies from each treatment group were embedded in epoxy and finely polished to expose skeletal cross-sections for quantitative elemental spot analysis. Prior to and following each run, the instrument was calibrated using repeat measurements of calcite and celestine standards. Software (PeakSight 4.0) determined standard-based corrections were then applied to the data, and drift offsets subtracted. Average skeletal concentrations of Mg and Ca (mol%) were determined for two to five zooids in each colony by analysing an average of nine 1 µm spots (15 kV, 20 nA beam) distributed across both the centres and margins of the transverse lateral zooecial walls of each zooid (see the electronic supplementary material).

In total, Mg and Ca concentrations were measured from 103 zooids across treatment groups. Standard deviations in the reproducibility of each spot measurement were calculated via software algorithm (PeakSight v. 4.0) and experiment-wide error for elemental measurements was estimated as the standard deviation fraction of total concentration element recorded. This percentage was calculated for each measurement, and then averaged across the full dataset generating average error estimates for Mg and Ca observations of 1.8% and 1.4%, respectively. Statistical comparisons were performed on molar Mg/CaC of zooid calcite [3] using a linear mixed effects model implemented in JMP PRO 12 (see the electronic supplementary material for further detail). We further quantified the percentage of observed points in each zooid falling above Mg/CaC of 0.12 (and thus likely matching or exceeding the solubility of aragonite) to compare variation in the occurrence of elevated Mg/CaC among treatments.

To verify that dissolved Ca and Mg concentrations in our experimental containers did not differ significantly among treatment groups, we determined the dissolved concentrations of these metals in seawater from a random sample of experimental containers each week, using an inductively coupled plasma mass spectrometer to analyse seawater samples. See the electronic supplementary material for results and details on chemical monitoring during the experiment.

(d) Image and statistical analyses

From our photo record of colony development, we used image analysis software (ImageJ v. 1.48) to quantify the number of three ontogenetic stages in J. tuberculata colonies at the beginning and end of the eight-week treatment phase. Zooids were identified as (i) immature zooids (zooids that were still forming at the growing edges of colonies when the image was taken), (ii) feeding autozooids (with a gut and a well-developed lophophore possessing a full complement of tentacles), and (iii) degenerated autozooids (indicated by the breakdown of the lophophore and gut, leaving a darkened remnant of cells referred to as a brown body).

We statistically modelled the growth efficiency (GE) of colonies as log10(Nn/Na), where Nn is the number of new zooids of all types added during the treatment phase, and Na is the number of feeding autozooids at the beginning of the treatment phase with temperature, CO2 and food level treated as fixed factors in a mixed-model design implemented in JMP Pro 12 (see the electronic supplementary material for further model detail). GE represents a metric for growth productivity, accounting for the initial resource-generating ‘population’ of zooids [25]. We additionally modelled the number of new degenerated zooids observed in colonies, calculated by subtracting the number of degenerated zooids observed in colonies prior to treatment exposures from the number observed at the end of the treatment phase. To visualize this latter relationship, we regressed the number of new degenerated zooids observed in colonies against the number of new zooids added, comparing regression slopes across treatment groups. Challenging environmental conditions are thought to trigger the degeneration of active zooids as a means of energy conservation [35]. Thus, we interpret increases in the slope of this relationship (i.e. higher numbers of degenerated zooids observed relative to the numbers of new zooids generated) to indicate a compensatory trade-off, where growth of a colony is occurring at the expense of zooid function and/or longevity. While it is possible that some zooids degenerated and then re-generated during the course of the experiment, we assume that the patterns quantified from images taken at the beginning and end of the experiment are an accurate reflection of compensatory shifts that occurred throughout this period.

The dissolution of zooid calcification during the study was apparent in our photo record. In this species, mature zooids typically possess large prominent calcified structures (termed tubercles, see electronic supplementary material, figure S1). Using our photo records of colony growth, we randomly selected 30 pre-treatment zooids and 30 mature treatment phase zooids from each colony used in our skeletal magnesium analysis. For each zooid, we recorded whether its surface exhibited: (i) no observable dissolution, (ii) active dissolution as indicated by pitting and depressions in calcified structures, or (iii) the disappearance of tubercle calcification under strong dissolution (electronic supplementary material, figure S1). We then compared dissolution of these colonies in separate analyses for zooids generated during the pre-treatment and the treatment phases using a linear mixed effects model (JMP Pro v. 12).

(b) Initial condition of bryozoan colonies

At the beginning of the treatment phase, there were no significant differences in the total number of zooids among colonies assigned to each of the eight treatment combinations (one-way ANOVA, F7,63 = 1.402, p = 0.223), nor were there differences in the proportion of degenerated versus active zooids (one-way ANOVA, F7,63 = 0.701, p = 0.670). Zooids generated prior to the treatment phase did not exhibit signs of dissolution.

(c) Growth efficiency and colony degeneration

GE increased with elevated food (p = 0.007), but GE was not significantly affected by temperature, CO2 or interactions among these factors (figure 1a). Variance components associated with genotype explained 38.82% of total variance in the GE model; however, no significant differences were observed among genotypes.

Growth efficiency (GE) and the number of new degenerated zooids relative to new zooids added in colonies. (a) Elevated food increased GE across treatment groups (p = 0.007), but GE was found to be equivalent across temperature and CO2 levels. Error bars are ±1 s.e. (b) The number of new degenerated zooids observed relative to the number of new zooids added by colonies increased under cold temperatures (p < 0.0001), high CO2 (p = 0.006) and the interaction of temperature × CO2 × food × new zooids added (p = 0.009). Shared letters indicate slope relationships among treatment groups that are not significantly different from each other (p > 0.05). This pattern suggests that colonies are able to maintain equivalent growth efficiencies across a range of conditions by increasing the degeneration of autozooids and redirecting this energy to the growth of new zooids.

The number of newly degenerated zooids increased with the number of new zooids added during the treatment phase (p < 0.0001) and increased under colder temperature (p = 0.013; figure 1b). Zooid degeneration also varied with the interaction terms of temperature × new zooids (p < 0.0001), CO2 × new zooids (p = 0.006) and the four-way interaction of temperature × CO2 × food × new zooids (p = 0.009). The slope of the relationship between new degenerated zooids and new zooids added was significantly higher at 11°C, high food, high CO2 when compared with the majority of colonies held at 21°C and when compared with colonies held under 11°C, high food, low CO2 (figure 1b).

Box plots of Mg/CaC in bryozoan zooids exposed to experimental treatments. +/− indicate high and low-food groups, respectively. (a) Mg/CaC ratios in pre-treatment zooids initially generated under 16°C low CO2, but then subsequently exposed to altered temperatures, CO2 and food concentrations. Reduced Mg/CaC is observed under high CO2 in pre-treatment zooids exposed to 11°C and low-food concentrations. (b) Mg/CaC ratios in zooids generated during the treatment phase. Elevated Mg/CaC is observed in new zooids added under 21°C compared with 11°C. Shared letters indicate groups that are not significantly different from each other based on means comparisons of least-squares group estimates (p > 0.05) in separate analyses for pre-treatment and treatment phase zooids.

Despite this naturally occurring variation, strong differences in the skeletal mineralogy of treatment phase zooids were observed between temperature treatments (p < 0.0001), with zooids exhibiting 29% greater Mg/CaC at 21°C (μ = 0.130) compared with zooids cultured at 11°C under low CO2 conditions (μ = 0.101). Mg/CaC of treatment phase zooids within each temperature group did not differ across food or CO2 levels (figure 2b). The occurrence of measurements containing Mg/CaC in excess of 0.12 (likely corresponding to equivalent or higher solubility than aragonite) was 169% greater in new zooids grown at 21°C compared with 11°C (p < 0.0001, figure 3a), with 65% of Mg/CaC measurements falling at or above 0.12 at 21°C. Mg/CaC in pre-treatment zooids varied with exposure to treatment phase temperature (p = 0.001), treatment phase CO2 (p = 0.045), the interaction of treatment phase temperature × CO2 (p = 0.016) and the three-way interaction of treatment temperature × CO2 × food level (p = 0.031). Pre-treatment zooids exposed to high CO2 and low food at 11°C exhibited a 32.5% decrease in the mean Mg/CaC (p = 0.022) compared with the low CO2, low-food treatment at 11°C, with the mean Mg/CaC being reduced from 0.12 to 0.081. Pre-treatment zooids exposed to high CO2 and low food at 11°C also exhibited lower Mg/CaC when compared with all pre-treatment zooids exposed to 21°C (figure 2a; see electronic supplementary material, table S3 for means comparisons and full model output).

(e) Skeletal dissolution

Dissolution of treatment phase zooids was greater at higher temperature (p < 0.0001) and elevated CO2 (p < 0.0001), and varied with the interactions of temperature × CO2 (p < 0.0001), and temperature × food (p < 0.0001), as well as with the three-way interaction of temperature × CO2 × food (p < 0.0001; figure 3b). Dissolution was not observed in zooids held under low CO2. At 21°C/high CO2, 90.6% and 92.6% of zooids exhibited dissolution for low and high food groups, respectively. At 11°C/high CO2, 41% and 9.7% of zooids exhibited dissolution for low and high food groups, respectively (figure 3b). Under this treatment combination, there was suggestive evidence that the frequency of dissolution was higher under low-food conditions than high food (p = 0.077).

Dissolution of pre-treatment zooids was greater when colonies were exposed to high versus low CO2 conditions during the treatment phase (p < 0.0001). Treatment phase temperatures and food levels did not cause significant differences in the dissolution of pre-treatment zooids under high CO2 (figure 3b). Under high CO2 at 11°C, 98.9% and 87.1% or pre-treatment zooids exhibited dissolution under low and high food, respectively, compared with 97.7% and 96.6% at low and high food under 21°C treatments.

4. Discussion

In this study, we hypothesized that multiple oceanographic parameters would interact to affect the growth and skeletal mineralogy of the bryozoan J. tuberculata, a colonial marine invertebrate with a global geographical distribution that includes all of the World's warmer seas. Surprisingly, bryozoan clones maintained equivalent growth efficiencies across treatments, even under the combination of conditions that we expected to be stressful (cold temperatures and high CO2). However, this maintenance was associated with costs to zooid longevity, with greater zooid degeneration relative to the production of new zooids (figure 1b). This degeneration decreased in warm temperatures, concurrent with an increase in the mean GE (figure 1a). These results suggest that growth is supported in adverse environments (e.g. cold temperatures accompanied by high CO2) by an increase in polypide (lophophore and visceral mass) degeneration of existing zooids, with colonies translocating limited available energy from maintenance functions to support the production and development of new zooids [23,35,36].

Jellyella tuberculata exhibited striking dissolution under dissolved CO2 concentrations that are already experienced episodically by this species in the northern portion of its geographical range on the California coast during upwelling events. These conditions are expected to become increasingly common under projected global CO2 emissions scenarios [21]. However, we found that dissolution was also strongly modulated by temperature and food availability, which affect skeletal Mg content of this species. The amount of Mg incorporated into skeletal material appears to be weakly regulated in J. tuberculata, potentially reflecting the evolution of a generalist growth and calcification strategy in this species, and the family Membraniporidae more broadly, which may render it more vulnerable to OA than other taxa (see below for further discussion).

(a) High temperature and low-food exacerbate dissolution

Jellyella tuberculata appears to secrete a highly heterogeneous mixture of Mg calcite, indicated by the range of Mg/CaC observed among and within zooids sampled in this study. During the course of this study, the skeletal mineralogies of three zooids from a field-collected colony were analysed using the same methods applied to our experimental colonies. These results also indicated the presence of variable Mg calcite, with Mg/CaC measurements within and among zooids ranging from 0.111 to 0.160 (Swezey et al., unpublished data), consistent with our laboratory findings and indicating that our results are likely representative of natural trends in J. tuberculata calcite from the California coast. Because instrumental error was low throughout our analysis, this variation appears to be a feature of this and potentially other high-Mg secreting species. Within this natural variation, Mg/CaC increased with temperature (figures 2b and 3a), supporting latitudinal trends observed previously in some bryozoans [27].

We also observed an increase in the dissolution of zooids grown in our high temperature treatments (figure 3b). We surmise that this pattern is primarily driven by the formation of high-Mg calcite at warm temperatures, which is then susceptible to subsequent dissolution under high CO2. In our experiment, the mean aragonite saturation state was roughly equivalent across high CO2 treatments (Ωarag = 0.997 and Ωarag = 1.082 for 11°C and 21°C), yet we saw marked dissolution under 21°C when compared with 11°C. This trend suggests that an increase in the Mg content of zooids grown in the high temperature treatments was a dominant cause of skeletal dissolution observed in our experiment.

In addition to Mg/CaC changes associated with temperature, colonies in our low-temperature, low-food treatments exhibited elevated mean Mg/CaC and elevated Mg/CaC > 0.12 in both pre-treatment and treatment phase zooids added under low CO2 when compared with colonies receiving high food (figures 2 and 3a). While these trends were not statistically significant, they indicate that Mg regulation is likely under some level of metabolic control in this species, whereby colonies with an elevated energetic status are more capable of excluding Mg ions from the calcite lattice. Low-food clones held under high CO2 at 11°C exhibited a roughly fourfold increase in the frequency of dissolution of treatment phase zooids and a 12% increase in dissolution of pre-treatment zooids, compared with colonies from the high CO2, 11°C, high food group (figure 3b). This difference is likely driven by increasing Mg concentrations in skeletal calcite, but may also reflect a more general disruption of skeletal maintenance when food and energy are limited. This food effect was not apparent in 21°C treatments, where Mg/CaC > 0.12 and dissolution under high CO2 was elevated in all groups.

Morse et al. [1] noted that rising pCO2 should result in sequential dissolution of Mg calcite according to mineral stability, gradually leading to removal of the more soluble phases until the least soluble phases remain. Despite our observations of extensive dissolution under high CO2, significant reductions in the average Mg/CaC of zooids only occurred in pre-treatment zooids exposed to high CO2 and low food when colonies were held at 11°C, but not when exposed to 21°C (figure 2a). This pattern suggests that at 21°C, J. tuberculata may exhibit elevated calcification activity, continuing to biomineralize in the face of dissolution. By contrast, colonies exposed to high CO2 and low food do not appear to replace lost high-Mg calcite in pre-treatment zooids when held at 11°C. This outcome may reflect a diversion of investment from reparative calcification to new zooid growth under these strenuous conditions, leading to predicted sequential dissolution and reductions in Mg/CaC over time.

(b) Taxonomic patterns of mineralogical control

Bryozoans employ a broad array of mineralogical strategies, and occupy 63% of the theoretically estimated space available for biomineralization, a dimensionless calculation of how widely a group varies in skeletal composition relative to the total range of known mineralogical strategies available to calcifying species (see [26] for details on calculation). Relative to other bryozoan families that occupy an average of 11% of this total range, Membraniporidae has among the most variable skeletal compositions, occupying approximately 87% of the range calculated for all bryozoan taxa [26]. Values of 14 mol% MgCO3 have been recorded in J. tuberculata from Bermuda [37], where 21°C is a frequent sea surface temperature experienced in near shore habitats [38]. These MgCO3 values are among the highest recorded in all bryozoan taxa [27].

Why is mineralogical composition so variable both within and among species in this family compared with nearly all other bryozoan taxa? Unlike most bryozoan taxa, members of this family possess a broadly dispersing cyphonautes larval form, spending two weeks to two months in the plankton [29,39], with concomitant high levels of genetic connectivity and a cosmopolitan global distribution [29,30,40]. Broad dispersal across multiple environments may therefore have selected for high levels of plasticity in skeletal mineralogy under spatially variable conditions. Adult colonies of the Membraniporidae also exhibit extremely rapid growth rates relative to other clades [23,41] and are dominant competitors for space on ephemeral habitat [23,42]. At our Dillon Beach collection site, this species consistently overgrows all other locally occurring encrusting bryozoan species (D.S.S. et al. 2013, personal observation). Rapid growth rates associated with this competitive dominance may have selected for reduced regulatory investment in calcite homogeneity and stability in favour of accelerated growth under a variety of environmental conditions. Indeed, this species maintains high growth rates even under adverse conditions at the expense of zooid (and potentially eventual colony) longevity. These hypothesized trade-offs among growth, longevity and plasticity in skeletal mineralogy might also lead to pronounced dissolution and morphological changes under increasing temperature and dissolved CO2 concentrations.

(c) Vulnerability in a coastal upwelling region

To the best of our knowledge, J. tuberculata does not occur along the California coast north of our study region of Bodega Bay ([30], D.S.S. et al. 2013, personal observation). Areas north of this location have been identified as centres of persistent coastal upwelling, where cold, low saturation state conditions prevail during the spring and summer months [17–19]. Our results suggest that J. tuberculata is mismatched with conditions in the upwelling zone of northern California. We hypothesize that J. tuberculata weakly regulates mineralogical composition, which may limit its northern distribution to Bodega Bay and locales south, and render this species more vulnerable to changing ocean conditions. In the case of J. tuberculata, temperatures and phytoplankton concentrations that prevail before the seasonal onset of upwelling may determine how susceptible colonies will be to dissolution when exposed to corrosive waters. Dissolution of skeletal material under recently intensified upwelling conditions along the west coast of North America has occurred in other taxa (e.g. pteropods; [43]), and consistent with our laboratory results, J. tuberculata colonies exhibit signs of skeletal dissolution in the field during periods of upwelling activity at Dillon Beach (D.S.S. et al. 2013, personal observation)

Observations and model projections suggest that climate change will increase the intensity of coastal upwelling in temperate latitudes [44–46], where upwelling seasons are projected to start earlier and end later [46]. Conversely, separate analyses have indicated that increased heat budgets due to elevated greenhouse gas concentrations may enhance stratification in the California Current System, deepening the thermocline and leading to higher temperatures in coastal waters [47,48]. Near shore habitats of coastal California have experienced significant warming in recent years [49,50] and alternations between warming and upwelling conditions [51] may deleteriously interact with the stability of calcification of some marine invertebrate species.

Skeletal dissolution in organisms that loosely regulate the Mg content of their calcified structures may be a widespread phenomenon that plays an important and underappreciated role in mediating the effects of OA. Reduced calcification rates and shifts in competitive hierarchies have been observed in other high-Mg calcite-secreting species such as coralline algae, concurrent with increasing OA [52]. Large-scale ecological impacts may therefore already be underway for some benthic marine communities. Quantifying the interactive effects of temperature, food availability and CO2 on growth and skeletal mineralogy in other potentially vulnerable high-Mg taxa should be a priority for future research.

Authors' contributions

D.S.S., J.R.B. and E.S. conceived the study, and all authors contributed to its design. D.S.S., J.R.B. and A.T.N. conducted the experiments and carried out analyses of water chemistry, the latter with input from T.M.H. D.S.S. conducted the statistical analyses, and all authors participated in the interpretation of data. D.S.S. and E.S. drafted the manuscript, and all authors contributed significantly to revisions. All authors gave final approval for publication.

Competing interests

We declare we have no competing interests.

Funding

This work was supported by grant nos. OCE-0927255, OCE-1041089, OCE-1220648 and DGE-0841297 from the National Science Foundation and the University of California Multicampus Research Programs and Initiatives (MRPI). D.S.S. was also supported by an NSF Graduate Research Fellowship.

Acknowledgements

We would like to thank G. Baxter, N. Botto, M. Carroll, K. Griffith, D. Hall, L. Heidenreich, T. Landers, J. Lankford, K. Laughlin, J. Newman, S. Roeske, L. Rose, N. Sippl-Swezey, P. Stull and T. Woods for their help with field and laboratory work and analyses during the course of this project. We would also like to thank N. Willits for statistical advice. This manuscript was improved by helpful comments from J. Stachowicz, R. Grosberg and two anonymous reviewers.

. 2011Interactions between an invasive and a native bryozoan (Membranipora membranacea and Electra pilosa) species on kelp and Fucus substrates in Nova Scotia, Canada. Mar. Biol.158, 2299–2311. (doi:10.1007/s00227-011-1734-3)